The present invention relates to a position detection apparatus that detects an absolute position of a movable member, and particularly to an optical position detection apparatus using an optical scale.
Optical position detection apparatuses are each constituted by a light-receiving sensor fixed to one of a movable member and a base member and an optical scale fixed to the other of the movable member and the base member. The optical scale is provided with a periodic pattern reflecting or transmitting light. Detecting the light from the periodic pattern by the light-receiving sensor with a movement of the movable member provides a detection signal changing at a period depending on a period of the periodic pattern.
Such optical position detectors (hereinafter referred to as “optical encoders”) include one having an optical scale provided with two (paired) periodic patterns whose phases are mutually different and detecting lights from the two periodic patterns by a light-receiving sensor to provide two periodic signals (paired two-phase signals) whose phases are mutually different. A calculation using the two-phase signals provides a relative position of the movable member relative to the base member.
Furthermore, the optical encoders include, as disclosed in Japanese Patent Laid-Open No. 2013-234861, another one using an optical scale provided with multiple paired periodic patterns each pair of which includes periodic patterns having a long period and a short period; the long periods and the short periods of the respective paired periodic patterns are mutually slightly different. This optical encoder detects lights from the multiple paired periodic patterns by a light-receiving sensor to generate multiple paired two-phase signals and performs calculations on the multiple paired two-phase signals to produce multiple position signals whose periods are mutually different (for example, an upper level signal having a long period and a lower level signal having a short period). Then, combining these multiple position signals enables calculating an absolute position of a movable member.
However, the above optical encoders have a problem that an unclean optical scale having a smudge (including a scratch) or dust thereon causes a noise in the detection signal, which disables an accurate position detection of the movable member, that is, causes erroneous position detection. In particular, in the optical encoder disclosed in Japanese Patent Laid-Open No. 2013-234861, a multiplying process performed on the upper level signal for combining the upper and lower level signals also amplifies the noise. Thus, the unclean optical scale significantly affects the position detection.
Furthermore, the above optical encoders have another problem that an expansion and a contraction of the optical scale due to a temperature change changes a relative positional relation between the movable member and the optical scale, which disables an accurate position detection of the movable member, that is, causes erroneous position detection. In particular, the optical encoder disclosed in Japanese Patent Laid-Open No. 2013-234861 performs the multiplying process performed on the upper level signal for combining the upper and lower level signals, so that a variation of the upper level signal due to the expansion and contraction of the optical scale significantly affects the position detection.
Moreover, in a rear-focus zoom lens in which its focus lens is disposed on an image plane side further than its magnification-varying lens, a movement of the magnification-varying lens causes an image plane movement. Thus, a control causing the focus lens to move with the movement of the magnification-varying lens so as to reduce the image plane movement, that is, a zoom tracking control is performed to maintain an in-focus state. An optical apparatus disclosed in Japanese Patent Laid-Open No. 06-205259 holds cam data indicating positions of the focus lens at which an in-focus state is maintained for respective positions of the magnification-varying lens and performs, by using the cam data, the zoom tracking control to move the focus lens with the movement of the magnification-varying lens.
This zoom tracking control is also performed when a manual zoom operation is performed for moving the magnification-varying lens through a user's rotation operation of a zoom operation ring. The manual zoom operation uses a magnification-varying mechanism that transmits the rotation of the zoom operation ring to a cam ring to rotate it about an optical axis and moves the magnification-varying lens in an optical axis direction by a cam formed in the cam ring. With the movement of the magnification-varying lens, the zoom tracking control detects rotation direction and position of the zoom operation ring, which correspond to a position of the magnification-varying lens, by a position detection apparatus such an encoder and moves the focus lens to a position in the cam data corresponding to the detected rotation direction and position.
Such a magnification-varying mechanism has an engagement portion in which the zoom operation ring and the cam ring engage with each other in order to transmit the rotation of the zoom operation ring to the cam ring, and the engagement portion has backlash. This backlash retards the rotation of the cam ring and thereby the magnification-varying is not moved until the backlash is reduced (eliminated). Furthermore, this retardation of the rotation of the cam ring until the backlash is reduced is generated when the zoom operation ring is rotated in both a telephoto direction and a wide-angle direction, so that two rotation positions of the zoom operation ring in each of a telephoto area and a wide-angle area are detected as the same rotation position. Accordingly, it is necessary to provide separate cam data for each rotation direction of the zoom operation ring, that is, for each magnification variation direction and to select the cam date used for the zoom tracking control, depending on the rotation direction of the zoom operation ring detected by using the position detection apparatus.
However, though the manual zoom performed by the user's rotation operation of the zoom operation ring can move the magnification-varying lens even in a non-energized state (power-off state) where the position detection apparatus is not energized, the rotation direction of the zoom operation ring cannot be detected. An unknown rotation direction of the zoom operation ring in the non-energized state makes the direction (telephoto or wide-angle direction) in which the backlash is reduced unclear at start of energization (power-on), which makes unclear which one of the cam data provided for the respective magnification variation directions should be selected immediately after the power-on. In addition, the unknown rotation direction of the zoom operation ring in the non-energized state makes unclear whether to immediately move the focus lens, in response to the rotation of the zoom operation ring after the power-on, according to the cam data provided for the rotation direction of the zoom operation ring (that is, the magnification variation direction) or to stop moving the focus lens until the backlash is reduced. This makes it impossible to perform a good zoom tracking control.